If you stretch your pecs when you train your lats you'll manage more reps

Strength athletes will get more reps out of their sets if they stretch their muscles between sets. And we're not talking about the muscles you're training, but their antagonists. So when you're training the latissimi dorsi you need to stretch the pectoralis major. Sports scientists at the Federal University of Rio de Janeiro write about it in Research in Sports Medicine.

Study
The Brazilians got 10 well-trained strength athletes to train their lats on two occasions by doing seated rows. The subjects did 3 sets using a weight with which they could manage 10 reps. Between sets they rested for two minutes.

On one occasion the subjects did nothing between sets. [Passive recovery] On the other occasion they stretched their pecs before starting the sets, doing the exercise shown below [Antagonist stretching]. The athletes did passive stretching, and held the stretch for 40 seconds.

Results
Via electrodes that they had attached to the subjects' muscles, the Brazilians could see that passive stretching of the pecs increased the electrical activity in the lats and the biceps during the sets. So the muscles were working harder. The stretching had no effect on the electrical activity in the pecs themselves.

The effect of stretching is obvious, and indeed, the subjects completed significantly more reps as a result.

The Brazilians don't know how stretching the pecs strengthens the lats.

Conclusion
"The antagonist stretching applied during the inter-set rest period showed greater repetition performance and agonist muscle activation in the seated row exercise compared with a passive rest period between the three sets", the researchers concluded.

"These results indicated an interesting alternative for coaches and practitioners during resistance training prescription aiming to enhance strength performance and muscle activation in an acute manner."

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